X-Rays Signal Presence of Elusive Intermediate Mass Black Hole

Peculiar outbursts of X-rays coming from a black hole have provided
evidence that it has a mass of about 10,000 Suns, which would place it
in a possible new class of black holes. The timing and regularity of
these outbursts, observed with NASA's Chandra X-ray Observatory, make
the object one of the best candidates yet for a so-called
intermediate-mass black hole.

Scientists have strong evidence for the existence of stellar black holes
that are about 10 times as massive as the Sun. They have also discovered
that supermassive black holes with masses as large as billions of Suns
exist in the centers of most galaxies. Recent evidence has suggested
that a new class of black holes may exist between these extremes –
intermediate-mass black holes with masses equal to thousands of Suns.

"It is important to verify the existence of intermediate-mass black
holes, because they would bridge the gap between stellar-mass black
holes and supermassive black holes in the centers of galaxies," said
Jifeng Liu of the University of Michigan in Ann Arbor, and lead author
on a paper describing their discoveries that appeared in the March 1
issue of the Astrophysical Journal Letters. "Our observations don't
settle the debate, but the behavior of this object is strong evidence in
favor of their existence."

Liu and his colleagues used Chandra to observe a black hole in the
galaxy Messier 74 (M74), which is about 32 million light years from
Earth. They found that this source exhibits strong, nearly periodic
variations in its X-ray brightness every two hours, providing an
important clue to the black holes' mass. The black hole also fell into a
class of sources called ultraluminous X-ray sources (ULXs) because they
radiate 10 to 1000 times more x-ray power than neutron stars and stellar
mass black holes.

Some astronomers believe these mysterious ULXs are more powerful because
they are intermediate mass black holes. Others think ULXs are regular
stellar-mass black holes that appear to be much more powerful in X-rays
because their radiation is beamed in a jet toward Earth.

Chandra's discovery of the persistence and long time period of the X-ray
variations (called quasi-periodic oscillations, because they are not
strictly periodic) of the ULX in M74 is an argument against a beamed
jet. These variations are likely produced by changes in a disk of hot
gas around the black hole. More massive black holes have larger disks,
which in turn are expected to vary over longer periods.

Independent observations of a wide range of black hole X-ray sources
with masses ranging from ten to tens of millions solar masses have
revealed a relationship between the time scale of quasi-periodic
oscillations and the mass of the underlying black hole. Using this
technique, the observed two-hour variation implies that this ULX has a
mass of about 10,000 Suns.

Such a large mass would place this black hole well above the
stellar-mass black hole limit of a few dozen solar masses. How then did
it form? The leading theories under consideration are that
intermediate-mass black holes form as dozens or even hundreds of black
holes merge in the center of a dense star cluster, or that they are the
remnant nuclei of small galaxies that are in the process of being
absorbed by a larger galaxy.

Chandra observed M74, which is in the constellation of Pisces, twice:
once in June 2001 and again in October 2001. The European Space Agency's
XMM-Newton satellite also observed this object in February 2002 and
January 2003. Other authors on the research paper are Joel Bregman, Ed
Lloyd-Davies, Jimmy Irwin, Catherine Espaillat, and Patrick Seitzer, all
of the University of Michigan in Ann Arbor. Miriam Krauss (Massachusetts
Institute of Technology), Roy Kilgard (Univ. of Leicester and
Smithsonian Astrophysical Observatory) and their colleagues have also
reported extreme variability and the presence of QPOs for this object.